1. Technical Field
The present invention relates to a semiconductor substrate and a method for producing a semiconductor substrate.
2. Related Art
Silicon carbide (SiC) which is a wide band gap semiconductor has a band gap twice or more than twice as wide as that of silicon (Si) in the related art and has attracted attention as a material for high-pressure resistant devices. Since the crystal formation temperature of SiC is higher than that of Si, it is difficult to form a single crystal ingot by a pulling method from a liquid phase, and a single crystal ingot is formed by a sublimation method. However, it is very difficult to form a SiC substrate having a large diameter and few crystal defects by a sublimation method. Therefore, the diameter of a currently available SiC substrate is from 3 to 4 inches, and also the price thereof is very high.
As the types of SiC, there are cubic silicon carbide (3C-SiC), hexagonal silicon carbide (4H-SiC, 6H-SiC), and the like divided depending on the crystal structure. Among these, SiC having a cubic crystal structure (3C-SiC) can be formed at a relatively low temperature, and can be epitaxially grown directly on a Si substrate. Therefore, a heteroepitaxial technique in which the crystal of 3C-SiC is grown on a surface of a Si substrate as a method for increasing the diameter of a SiC substrate has been studied. However, the lattice constants of Si and 3C-SiC are 0.543 nm and 0.436 nm, respectively, and there is an about 20% difference therebetween. Further, Si and 3C-SiC are also different in thermal expansion coefficient, and therefore, it is difficult to obtain a high-quality epitaxial film with few crystal defects.
Techniques for solving such a problem have been studied. For example, JP-A-11-181567 describes the following technique. After a mask layer is formed on a surface of a substrate for use in the growth of silicon carbide, openings are formed in the mask layer so as to expose portions of the surface of the substrate, and single crystal silicon carbide is epitaxially grown. The height of each opening is set to a value 21/2 times or more the width of the opening and also exceeding the thickness of single crystal silicon carbide to be formed.
However, in a process in which single crystal silicon carbide is epitaxially grown and the temperature is returned to room temperature, it is difficult to suppress stress caused by a difference in thermal expansion coefficient between the substrate and the single crystal silicon carbide. If stress is caused by a difference in thermal expansion coefficient, crystal defects may occur in a single crystal silicon carbide film.